Optical beam scanning apparatus and image forming apparatus

ABSTRACT

In an image forming apparatus provided with an optical beam scanning apparatus according to the invention, an optical beam scanning apparatus of an overillumination scanning optical system includes a semiconductor laser device as a light source, a pre-deflection optical system, a polygon mirror, and a post-deflection optical system, with a width of the luminous flux made incident on the polygon mirror being wider than a width of one reflecting surface forming the polygon mirror, wherein at least two sheets of flat plate for transmitting the luminous flux scanned by the polygon mirror are provided in the post-deflection optical system. In accordance with an image forming apparatus provided with an optical beam scanning apparatus according to the invention, not only a wave front aberration on a photoconductive drum can be suitably corrected, but suitable beam diameter and beam profile can be obtained on the photoconductive drum.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of application Ser. No. 11/694,537filed Mar. 30, 2007, the entire contents of which is incorporated hereinby reference.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to an optical beam scanning apparatus andan image forming apparatus provided with this optical scanningapparatus. In particular, the invention relates to an optical beamscanning apparatus which in an overillumination scanning optical systemin which a width of an luminous flux made incident on a polygon mirroris wider than a width of one reflecting surface forming the polygonmirror, is able to scan the luminous flux on a photoconductive drum andan image forming apparatus provided with this optical beam scanningapparatus.

2. Related Art

In recent years, in image forming apparatus of an electrophotographicmode, for example, laser printers, digital copiers and laser facsimiles,an optical beam scanning apparatus for irradiating laser light (opticalbeam) on a surface of a photoconductive drum and scanning the laserlight to form an electrostatic latent image on the photoconductive drumis provided.

Recently, in order to devise to realize high-speed scanning on a surfaceof a photoconductive drum, for example, a method in which plural lightsources (laser diodes) are provided in one laser unit, therebyincreasing the number of laser light (multibeam mode) is proposed. Inthis multibeam method, plural beams for every color component emittedfrom each of light sources (for example, yellow, magenta, cyan, andblack) are processed in a pre-deflection optical system and convertedinto one beam, which is then made incident on a polygon mirror. The beamdeflected by the polygon mirror is mediated through an fθ lensconfiguring a post-deflection optical system and subsequently separatedinto a beam for every color component and irradiated on aphotoconductive drum of every color component.

Here, the rotation axis direction of the polygon mirror as a deflectoris defined as “sub-scanning direction”, and a direction vertical to eachof the optical axis direction of the optical system and the rotationaxis direction of the deflector (polygonal mirror) is defined as “mainscanning direction”. Incidentally, the sub-scanning direction in theoptical system is corresponding to a conveyance direction of a transfermaterial in an image forming apparatus, and the main scanning directionin the optical system is corresponding to a direction vertical to theconveyance direction within a surface of the transfer material in theimage forming apparatus. Also, an image surface shows the surface of thephotoconductive drum, and an imaging surface shows a surface on which aluminous flux (laser light) actually forms an image.

In general, a relation expressed by [Expression 1] is present among animage processing rate (paper conveyance rate), an image resolution, amotor rotation rate and a number of polygon mirror surfaces.

$\begin{matrix}{{P*R} = \frac{25.4*{Vr}*N}{60}} & \left\lbrack {{Expression}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In the foregoing expression, P (mm/s) represents a processing rate(paper conveyance rate); and R (dpi) represents an image resolution(number of dots per inch). Also, Vr (rpm) represents a number ofrevolutions of polygon motor; and N represents a number of polygonmirror surfaces.

As expressed by the foregoing [Expression 1], the printing speed andresolution in the image forming apparatus are proportional to the numberof revolutions of polygon motor (Vr) and the number of polygon mirrorsurfaces (N). Accordingly, in order to realize high resolution as wellas high speed in the image forming apparatus, it is necessary toincrease the number of polygon mirror surfaces (N) or to raise thenumber of revolutions of polygon motor (Vr).

However, in a conventional general underillumination scanning opticalsystem, a width of a luminous flux (laser light) made incident on apolygon mirror in a main scanning direction is made smaller than a widthof one reflecting surface forming the polygon mirror in the mainscanning direction (reflection width) thereby reflecting the whole ofthe luminous flux (laser light) made incident on the polygon mirror.

However, since not only a beam diameter on the image surface isproportional to an F number, but also the F number is expressed byFn=f/D wherein f represents a focal distance of the imaging opticalsystem, and D represents a beam diameter of the main scanning directionon the polygon mirror surface, when it is intended to make the beamdiameter on the image surface small for the purpose of devising torealize high image quality, the beam diameter of the main scanningdirection on the polygon mirror surface must be made large.

In other words, in order to obtain high image quality at a certain fixedlevel or more, there is present a restriction that the beam diameter ofthe main scanning direction on the polygon mirror surface must beregulated to a fixed size or more.

Nevertheless, in order to realize high resolution as well as high speed,when it is intended to increase the number of polygon mirror surfaces(N), the polygon mirror itself must be increased in size. As a result,when it is intended to rotate a large-sized polygon mirror at a highspeed, a load to a motor for driving the polygon mirror becomes large,and the motor cost increases. In addition, at the same time, the noiseor vibration of the motor or the generation of a heat becomes large, anda countermeasure thereto becomes necessary separately.

Then, an image forming apparatus using an over-illumination scanningoptical system is proposed in place of the underillumination scanningoptical system. In the overillumination scanning optical system, a widthof a luminous flux made incident on a polygon mirror is made wider thana width of one reflecting surface forming the polygon mirror.

According to this, it is possible to reflect the luminous flux by usingthe entire surface of the reflecting surface forming the polygon mirror(or plural reflecting surfaces); and even in the case where it isintended to ensure the beam diameter on the polygon mirror surface whileincreasing the number of reflecting surfaces of polygon mirror (N) forthe purpose of devising to realize high resolution as well as highspeed, it is possible to make the diameter of the polygon mirror itselfsmall. Accordingly, a load to a motor for driving the polygon mirror canbe reduced, and the motor cost can be reduced. Also, since not only thediameter of the polygon mirror itself can be made small, but also thenumber of reflecting surfaces can be increased, it is possible to makethe shape of the polygon mirror close to a circle, and it is possible tomake the air resistance at the time of driving the polygon mirror low.As a result, even when the polygon mirror is rotated in a high speed, itis possible to reduce the noise or vibration and the generation of aheat.

Furthermore, following the reduction in the noise or vibration and thegeneration of heat, the whole or a part of countermeasures parts forreducing the noise or vibration, such as glasses, become unnecessary,and the costs in manufacturing an image forming apparatus can belowered. Also, a high duty cycle becomes possible.

The foregoing overillumination scanning optical system is described in,for example, Leo Beiser, Laser Scanning Notebook, SPIE OPTICALENGINEERING PRESS.

In general, in scanning a luminous flux made incident from asemiconductor laser device on a photoconductive drum, in theunderillumination scanning optical system, an edge part of a polygonmirror is not used, whereas in the overillumination scanning opticalsystem, an edge part of a polygon mirror is used.

For that reason, since an error in the shape in the edge part of themain scanning direction of the polygon mirror is large, when a luminousflux is deflected (scanned) by using the edge part, the luminous flux tobe deflected (scanned) becomes dull, whereby a wave front aberration isdeteriorated in the entire scanning region on the photoconductive drum.As a result, the beam diameter on the photoconductive drum as an imagesurface increases, whereby a beam profile is deteriorated.

SUMMARY OF THE INVENTION

In view of such circumstances, the invention has been made and is aimedto provide an optical beam scanning apparatus in which not only a wavefront aberration on a photoconductive drum can be suitably corrected,but suitable beam diameter and suitable beam profile can be obtained onthe photoconductive drum and an image forming apparatus provided withthis optical beam scanning apparatus.

In order to solve the foregoing problems, an optical beam scanningapparatus according to an aspect of the invention is an optical beamscanning apparatus including a light source configured to emit aluminous flux, a pre-deflection optical system configured to form aluminous flux emitted from the light source to image the luminous fluxas a line image in a prescribed direction corresponding to a mainscanning direction, a scanning unit configured to scan the imagedluminous flux by the pre-deflection optical system against a scanningsubject, and a post-deflection optical system configured to image theluminous flux scanned by the scanning unit on the scanning subject, inwhich a width of the luminous flux made incident on the scanning unitfrom the pre-deflection optical system is wider than a width of onereflecting surface forming the scanning unit, wherein at least twosheets of flat plate configured to transmit the luminous flux scanned bythe scanning unit are provided in the post-deflection optical system.

In order to solve the foregoing problems, an image forming apparatusaccording to an aspect of the invention is an image forming apparatusprovided with an optical beam scanning apparatus including a lightsource configured to emit a luminous flux, a pre-deflection opticalsystem configured to form a luminous flux emitted from the light sourceto image the luminous flux as a line image in a prescribed directioncorresponding to a main scanning direction, a scanning unit configuredto scan the imaged luminous flux by the pre-deflection optical systemagainst a scanning subject, and a post-deflection optical systemconfigured to form the luminous flux scanned by the scanning unit on thescanning subject, in which a width of the luminous flux made incident onthe scanning unit from the pre-deflection optical system is wider than awidth of one reflecting surface forming the scanning unit, wherein atleast two sheets of flat plate configured to transmit the luminous fluxscanned by the scanning unit are provided in the post-deflection opticalsystem.

In the optical beam scanning apparatus according to an aspect of theinvention, in the optical beam scanning apparatus including a lightsource configured to emit a luminous flux, a pre-deflection opticalsystem configured to form a luminous flux emitted from the light sourceto image the luminous flux as a line image in a prescribed directioncorresponding to a main scanning direction, a scanning unit configuredto scan the imaged luminous flux by the pre-deflection optical systemagainst a scanning subject, and a post-deflection optical systemconfigured to image the luminous flux scanned by the scanning unit onthe scanning subject, in which a width of the luminous flux madeincident on the scanning unit from the pre-deflection optical system iswider than a width of one reflecting surface forming the scanning unit,at least two sheets of flat plate configured to transmit the luminousflux scanned by the scanning unit are provided in the post-deflectionoptical system.

In the image forming apparatus according to an aspect of the invention,in image forming apparatus provided with an optical beam scanningapparatus including a light source configured to emit a luminous flux, apre-deflection optical system configured to form a luminous flux emittedfrom the light source to image the luminous flux as a line image in aprescribed direction corresponding to a main scanning direction, ascanning configured to scan the imaged luminous flux by thepre-deflection optical system against a scanning subject, and apost-deflection optical system configured to image the luminous fluxscanned by the scanning unit on the scanning subject, in which a widthof the luminous flux made incident on the scanning unit from thepre-deflection optical system is wider than a width of one reflectingsurface forming the scanning unit, at least two sheets of flat plateconfigured to transmit the luminous flux scanned by the scanning unitare provided in the post-deflection optical system.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings,

FIG. 1 is a view to show a configuration of an image forming apparatusprovided with an optical beam scanning apparatus according to theinvention;

FIG. 2 is a view to show a detailed configuration of the optical beamscanning apparatus of FIG. 1;

FIG. 3 is a view to show an outline configuration of the inside of acontrol system of an image forming apparatus provided with the opticalbeam scanning apparatus of FIG. 2;

FIG. 4 is a diagram to show respective factors of a surface shape of animaging lens;

FIG. 5 is a diagram to show a detailed configuration of an optical beamscanning apparatus in the case of using an imaging mirror;

FIG. 6 is an explanatory view to explain an imaging lens including asurface having a diffraction surface;

FIG. 7 is an explanatory view to explain other imaging lens including asurface having a diffraction surface;

FIG. 8 is an explanatory view to explain an imaging lens where onlycurvatures of a lens are arranged;

FIG. 9 is an explanatory view to explain other imaging lens including asurface having a diffraction surface;

FIG. 10 is an outline view to show a reflection position on a polygonmirror surface of an underillumination scanning optical system or anoverillumination scanning optical system;

FIG. 11 is an explanatory view to explain a flat plate glass providedbetween a polygon mirror and an imaging lens particularly in apost-deflection optical system;

FIG. 12 is an explanatory view to explain a correction method forcorrecting a wave from aberration on a photoconductive drum according tothe invention;

FIG. 13 is a diagram to show a simulation result in the case ofcorrecting a wave front aberration on a photoconductive drum byemploying a correction method for correcting a wave from aberration on aphoto-conductive drum according to the invention; and

FIG. 14 is an explanatory diagram to explain an inclination method forinclining a flat plate glass around a main scanning direction axisagainst an optical axis in correcting a wave front aberration on aphotoconductive drum according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the invention are hereunder described with reference tothe drawings.

FIG. 1 shows a configuration of an image forming apparatus 1 providedwith an optical beam scanning apparatus 21 according to the invention.

As illustrated in FIG. 1, the image forming apparatus 1 includes, forexample, a scanner section 10 as an image reading unit and a printersection 20 as an image forming unit.

The scanner section 10 has a first carriage 11 formed movably into anarrow direction; a second carriage 12 to be moved following the firstcarriage 11; an optical lens 13 for imparting a prescribed imagingcharacteristic to light from the second carriage 12; a photoelectricconversion device 14 for not only photoelectrically converting the lightto which a prescribed imaging characteristic has been imparted by theoptical lens 13 but outputting an electric signal after thephotoelectric conversion; an original table 15 for holding an originalD; an original fixing cover 16 for fixing the original D by pressing itonto the original table 15, and the like.

The first carriage 11 is provided with a light source 17 forilluminating the original D and a mirror 18 a for reflecting catoptriclight reflected from the original D upon illumination with light emittedfrom the light source 17 towards the second carriage 12.

The second carriage 12 has a mirror 18 b for bending light guided fromthe mirror 18 a of the first carriage 11 by 90° and a mirror 18 c forfurther bending the light which has been bent by the mirror 18 b by 90°.

The original D placed on the original table 15 is illuminated by thelight source 17, thereby reflecting catoptric light where light andshade of light corresponding to the presence or absence of an image isdistributed. This catoptric light due to the original D is made incidentand guided as image information of the original D into the optical lens13 via the mirrors 18 a, 18 b and 18 c.

The catoptric light guided into the optical lens 13 from the original Dis collected on a light-receiving surface of the photoelectricconversion device (for example, a CCD sensor) 14 by the optical lens 13.

Then, when an indication to start the image formation is inputted from anon-illustrated operation panel or external apparatus, the firstcarriage 11 and the second carriage 12 are once moved to a home positionwhich is determined in advance so as to have a prescribed positionalrelation to the original table 15 by drive of a non-illustrated carriagedriving motor.

Thereafter, when the first carriage 11 and the second carriage 12 aremoved along the original table 15 at a prescribed rate, not only theimage information of the original D, namely the catoptric light (imagelight) reflected from the original D is cut out in a prescribed widthalong a direction where the mirror 18 a is extended, namely a mainscanning direction and reflected towards the mirror 18 b, but thecatoptric light reflected from the original D is successively taken outin units of a width cut out from the mirror 18 a with respect to adirection orthogonal to the direction where the mirror 18 a is extended,namely a sub-scanning direction. According to this, all the imageinformation of the original D is guided into the photoelectricconversion device 14. Incidentally, an electric signal outputted fromthe photoelectric conversion device 14 is an analogue signal and isconverted into a digital signal by a non-illustrated A/D converter andtemporarily stored as an image signal in a non-illustrated image memory.

Thus, the image of the original D placed on the original table 15 isconverted into a digital image signal of, for example, 8 bits exhibitinglight and shape of an image in a non-illustrated image processingsection for every one line along the first direction where the mirror 18a is extended by the photoelectric conversion device 14.

The printer section 20 has the optical beam apparatus 21 as an exposureapparatus as explained later by referring to FIG. 2 and FIG. 3 and animage forming section 22 of an electrophotographic mode capable offorming an image on recording paper P as a medium on which an image isformed.

The image forming section 22 is rotated by a main motor 23A such that anouter peripheral surface is moved at a prescribed rate as explained byreferring to FIG. 3. The image forming section 22 has a drum-likephotoconductor (hereinafter referred to as “photoconductive drum”) 23 onwhich an electrostatic latent image is formed corresponding to an imagedata, namely the image of the original D upon irradiation with a laserbeam (laser light) L from the optical beam scanning apparatus 21; acharging apparatus 24 for giving a surface potential of a prescribedpolarity to a surface of the photoconductive drum 23; a developmentapparatus 25 for selectively feeding a toner as a visualizing materialto the electrostatic latent image on the photoconductive drum 23 formedby the optical beam scanning apparatus and developing it; a transferapparatus 26 for giving a prescribed electric field to a toner imageformed on the outer periphery of the photoconductive drum 23 by thedevelopment apparatus 25 and transferring it onto the recording paper P;a separation apparatus 27 for releasing the recording paper P onto whichthe toner image has been transferred by the transfer apparatus 26 andthe toner between the recording paper P and the photoconductive drum 23from electrostatic adsorption with the photoconductive drum 23 andseparating them from the photoconductive drum 23; and a cleaningapparatus 28 for removing the transfer residual toner remaining on theouter peripheral surface of the photoconductive drum 23 and returningthe potential distribution of the photoconductive drum 23 to a statebefore the surface potential is fed by the charging apparatus 24; andthe like.

Incidentally, the charging apparatus 24, the development apparatus 25,the transfer apparatus 26, the separation apparatus 27 and the cleaningapparatus 28 are disposed in this order along an arrow direction inwhich the photoconductor drum 23 is rotated. Also, the laser beam L fromthe optical beam scanning apparatus 21 is irradiated in a prescribedposition X on the photoconductive drum 23 between the charging apparatus24 and the development apparatus 25.

In a non-illustrated image processing section, an image signal read fromthe original D in the scanner section 10 is not only converted into aprinting signal by processing, for example, contour correction orgradation processing for half tone display but converted into a lasermodulation signal for changing a light intensity of the laser beam Lemitted from a semiconductor laser device (semiconductor laser device 41of FIG. 2) of the optical beam scanning apparatus 21 as described laterto any one of an intensity at which an electrostatic latent image can berecorded on the outer periphery of the photoconductive drum 23 to whicha prescribed surface potential is given by the charging apparatus 24 oran intensity at which the latent image is not recorded.

Each semiconductor laser device (semiconductor laser device 41 of FIG.2) provided in the optical beam scanning apparatus 21 is subjected tointensity modulation based on the foregoing laser modulation signal andemits light so as to record an electrostatic latent image in aprescribed position of the photoconductive drum 23 corresponding to aprescribed image data. This laser light from the semiconductor laserdevice is deflected in a first direction which is the same direction asa reading line of the scanner section 10 by a deflector (polygon mirror50 as a deflector of FIG. 2) within the optical beam scanning apparatus21 and irradiated in the prescribed position X on the outer periphery ofthe photoconductive drum 23.

Then, when the photoconductive drum 23 is rotated in the arrow directionat a prescribed rate, similar to the movement of the first carriage 11and the second carriage 12 of the scanner section 10 along the originaltable 7, a laser beam from the semiconductor laser device which issuccessively deflected by the deflector (polygon mirror 50 as adeflector of FIG. 2) within the optical beam scanning apparatus 21 isexposed at prescribed intervals on the outer periphery of thephotoconductive drum 23 for every one line.

An electrostatic latent image corresponding to the image signal is thusformed on the outer periphery of the photoconductive drum 23.

The electrostatic latent image formed on the outer periphery of thephotoconductive drum 23 is developed with a toner from the developmentapparatus 25. A toner image developed with the toner is not onlyconveyed to a position opposing to the transfer apparatus 26 due to therotation of the photoconductive drum 23 but transferred onto therecording paper P which is fed by taking out a single sheet thereof froma paper cassette 29 by a paper feed roller 30 and a separation roller 31and then adjusting the timing by aligning rollers 32, due to an electricfield from the transfer apparatus 26.

The recording paper P onto which the toner image has been transferred isseparated together with the toner by the separation apparatus 27 andguided into a fixation apparatus 34 by a conveyance apparatus 33.

The recording paper P guided into the fixation apparatus 34 is subjectedto fixation of the toner (toner image) due to a heat and a pressure fromthe fixation apparatus 34 and then discharged into a tray 36 by paperdischarge rollers 35.

On the other hand, the photoconductive drum 23 in which the toner image(toner) has been transferred onto the recording paper P by the transferapparatus 26 is made opposed to the cleaning apparatus 28 due to thesubsequent continuous rotation. Then, the transfer residual toner(residual toner) remaining on the outer peripheral surface of thephotoconductive drum 23 is removed by the cleaning apparatus 28.Furthermore, the photoconductive drum 23 is returned to an initial statewhich is a state before feeding a surface potential by the chargingapparatus 24. According to this, next image formation becomes possible.

By repeating the foregoing process, a continuous image forming actionbecomes possible.

Thus, when the image information is read in the scanner section 10 andthe read image information is converted into a toner image in theprinter section 20 and outputted onto the recording paper P, theoriginal D set on the original table 15 is copied.

Incidentally, while the foregoing image forming apparatus 1 has beenapplied to a digital copier or the like, it is not limited to such case.For example, it may be applied to a printer apparatus in which an imagereading section is not present or the like.

FIG. 2( a) and FIG. 2( b) each shows a detailed con-figuration of theoptical beam scanning apparatus 21 of FIG. 1. Incidentally, FIG. 2( a)is an outline plan view in the case where plural optical elementsdisposed between a light source (semiconductor laser device 41) includedin the optical beam scanning apparatus 21 and the photoconductive drum23 (defined as “scanning subject”) are viewed from an orthogonaldirection (sub-scanning direction) to a main scanning direction which isa parallel direction to a direction in which laser light going from thepolygon mirror 50 as a deflector towards the photoconductive drum 23 isscanned by the polygon mirror 50. FIG. 2( b) is an outlinecross-sectional view of the optical beam scanning apparatus 21 on anX-X′ line of FIG. 2( a).

As illustrated in FIG. 2( a) and FIG. 2( b), the optical beam scanningapparatus 21 has a pre-deflection optical system 40 having thesemiconductor laser device 41 for emitting the laser beam (laser light)L of, for example, 658 nm; a collimation lens 42 for converting across-sectional beam shape of the laser beam L emitted from thesemiconductor laser device 41 into convergent light or parallel light ordivergent light; an aperture 43 for controlling the quantity of light(luminous flux width) of the laser beam L which has passed through thecollimation lens 42 to a prescribed size; a cylindrical lens 44 which isgiven a positive power only in the sub-scanning direction for thepurpose of arranging the cross-sectional shape of the laser beam L, thequantity of light of which has been controlled by the aperture 43, intoa prescribed cross-sectional beam shape; a mirror 45 for bending thelaser beam L from the semiconductor laser device 41, which has beenarranged into a prescribed cross-sectional beam shape by a finite focallens or the collimation lens 42, the aperture 43 and the cylindricallens 44, in a prescribed direction; and the like.

The polygon mirror 50 integrally formed with a polygon mirror motor 50Arotating at a prescribed rate is provided In a direction where the laserbeam L to which a prescribed cross-sectional beam shape has been givenby the pre-deflection optical system 40 advances. The polygon mirror 50scans the laser beam L, the cross-sectional beam shape of which has beenarranged into the prescribed shape by the cylindrical lens 44, towardsthe photoconductive drum 23 positioned at a later stage.

A post-deflection optical system 60 for imaging the laser beam L whichis continuously reflected on each of reflecting surfaces of the polygonmirror 50 in a generally straight line along an axis direction of thephotoconductive drum 23 is provided between the polygon mirror 50 andthe photoconductive drum 23. Incidentally, the “post-deflection opticalsystem” in the embodiment of the invention means all of optical systemsbetween the polygon mirror 50 and the photoconductive drum 23 andincludes an optical system between the polygon mirror 50 and an imaginglens 61 and an optical system between the imaging lens 61 and thephotoconductive drum 23.

The post-deflection optical system 60 is composed of the imaging lens(generally called as “fθ lens”) 61; a dustproof glass 62 for preventingturnaround of the toner, dusts or paper powder or the like floatingwithin the image forming section 22 into a non-illustrated housing ofthe optical beam scanning apparatus 21; and the like. The imaging lens61 is able to irradiate the laser beam L continuously reflected on theindividual reflecting surfaces of the polygon mirror 50 from one end tothe other end of the longitudinal (axis) direction of thephotoconductive drum 23 in the exposing position X as illustrated inFIG. 1 while making the position on the photoconductive drum 23proportional to a rotation angle of each of the reflecting surface ofthe polygon mirror 50 upon irradiation on the photoconductive drum 23and also to provide convergence properties to which a prescribedrelation based on an angle at which the polygon mirror 50 is rotated soas to have a prescribed cross-sectional beam diameter in any position ofthe longitudinal direction on the photoconductive drum 23.

Incidentally, an optical path of the laser beam L from the semiconductorlaser device 41 within the optical beam scanning apparatus 21 to thephotoconductive drum 23 is bent within a non-illustrated housing of theoptical beam scanning apparatus 21 by non-illustrated plural mirrors orthe like. Also, the imaging lens 61 and at least one non-illustratedmirror may be integrally formed in advance by optimizing curvatures ofthe imaging lens 61 in the main scanning direction and the sub-scanningdirection and an optical path between the polygon mirror 50 and thephotoconductive drum 23.

Also, in the optical beam scanning apparatus 21 as illustrated in FIG.2( a) and FIG. 2( b), when an axis O_(I) along a principal ray of thelaser beam L made incident on each of the reflecting surfaces of thepolygon mirror 50 and an optical axis O_(R) of the post-deflectionoptical system 60 are each projected on a main scanning plane on thephotoconductive drum 23, an angle α formed by the both is 5°, whereas ascanning angle β of a half-image region is 26°. Also, in the opticalbeam scanning apparatus 21 as illustrated in FIG. 2( a) and FIG. 2( b),an angle formed by the laser beam L made incident and the optical axisO_(R) of the post-deflection optical system 60 is 20.

Next, FIG. 3 shows an outline configuration of the inside of a controlsystem of the image forming apparatus 1 including the optical beamscanning apparatus 21 as illustrated in FIG. 2( a) and FIG. 2( b).

A CPU (central processing unit) 101 as a main control apparatus isconnected with a ROM (read only memory) 102 storing a prescribedoperation rule or initial data; a RAM (random access memory) 103 fortemporarily storing an inputted control data, a result of arithmeticprocessing by the CPU 101, or the like; an image RAM 104 for not onlyholding an image data from the photoelectric conversion device 14 or animage data fed from an external apparatus but outputting an image datato an image processing circuit 106; an NVM (non-volatile memory) 105 forholding a data which has been stored so far even in the case whereelectricity to the image forming apparatus 1 is blocked due to batterybackup; the image processing circuit 106 for subjecting the image datastored in the image RAM 104 to prescribed image processing, and thenoutputting it to a laser driver 121; and the like.

Also, the CPU 101 is connected with the laser driver 121 for making thesemiconductor laser device 41 of the optical beam scanning apparatus 21emit light; a polygon motor driver 122 for driving the polygon motor 50Afor rotating the polygon mirror 50; a main motor driver 123 for drivingthe main motor 23A for driving the photoconductive drum 23, a conveyancemechanism of the recording paper P or the like; and the like.

In the optical beam scanning apparatus 21, the divergent laser beam Lemitted from the semiconductor laser device 41 is converted intoconvergent light, parallel light or divergent light with respect to thecross-sectional beam shape by the lens 42.

The laser beam L, the cross-sectional beam shape of which has beenconverted into a prescribed shape, passes through the aperture 43,whereby not only the luminous flux width and the quantity of light areoptimally set up, but prescribed convergence properties are given onlyin the sub-scanning direction by the cylindrical lens 44. According tothis, the laser beam L becomes linear (line image) extending in the mainscanning direction on each of the reflecting surfaces of the polygonmirror 50.

The polygon mirror 50 is, for example, a regular dodecahedron and isformed so as to have an inscribed circle diameter Dp of about 25 mm.When the number of reflecting surfaces of the polygon mirror 50 isdefined as N, a width Wp of the main scanning direction of each of thereflecting surfaces (12 surfaces) of the polygon mirror 50 can bedetermined as expressed by [Expression 2].

Wp=tan(π/N)×Dp  [Expression 2]

In the case of the embodiment of the invention, the width Wp of the mainscanning direction of each of the reflecting surfaces (12 surfaces) ofthe polygon mirror 50 is Wp=tan(π/12)×25=6.70 mm.

On the other hand, a beam width D_(L) of the main scanning direction ofthe laser beam L irradiated on each of the reflecting surfaces of thepolygon mirror 50 is generally 32 mm and is set up widely as comparedwith the width Wp=6.70 mm of the main scanning direction of theindividual reflecting surfaces of the polygon mirror 50. By setting upthe beam width D_(L) of the main scanning direction of the laser beam Lwidely in the main scanning direction, it is possible to reducescattering in the quantity of light between the scanning end and thescanning center on the image surface (photoconductive drum 23).

The laser beam L which has been scanned (deflected) in a straight lineupon being guided onto each of the reflecting surfaces of the polygonmirror 50 and then continuously reflected due to the rotation of thepolygon mirror 50 is imparted a prescribed imaging characteristic by theimaging lens 61 of the post-deflection optical system 60 such that thecross-sectional beam diameter is generally uniform in at least the mainscanning direction on the photoconductive drum 23 (image surface) andimaged in a generally straight line on the surface of thephotoconductive drum 23.

Also, the rotation angle of the individual reflecting surfaces of thepolygon mirror 50 and the scanning position (imaging position) of thelight beam imaged on the photoconductive drum 23 are corrected by theimaging lens 61 so as to have a proportional relation with each other.Accordingly, the speed of the light beam which is scanned in a straightline on the photoconductive drum 23 becomes constant over the entirescanning region by the imaging lens 61. Incidentally, in the imaginglens 61, the respective reflecting surfaces of the polygon mirror 50 areindividually non-parallel to the sub-scanning direction, namely acurvature (curvature of the sub-scanning direction) capable ofcorrecting a deviation of the scanning position in the sub-scanningdirection due to an influence caused by the generation of inclination oneach of the reflecting surfaces is imparted. Furthermore, an imagesurface curve of the sub-scanning direction is corrected, too. In orderto correct these optical characteristics, the curvature of thesub-scanning direction is changed by the scanning position.

The shape of the lens surface of the imaging lens 61 has numericalvalues as shown in, for example, FIG. 4 and is defined according to[Expression 3].

$\begin{matrix}{X = {\frac{{{CUY}*y^{2}} + {{CUZ}*z^{2}}}{1 + \sqrt{\begin{matrix}{1 - {{AY}*{CUY}^{2}*y^{2}} -} \\{{AZ}*{CUZ}^{2}*z^{2}}\end{matrix}}} + {\sum\limits_{n = 0}\; {\sum\limits_{m = 0}\; {A_{mn}y^{m}z^{2n}}}}}} & \left\lbrack {{Expression}\mspace{14mu} 3} \right\rbrack\end{matrix}$

By using such imaging lens 61, the rotation angle θ of the individualreflecting surfaces of the polygon mirror 50 and the position of thelaser beam L to be imaged on the photoconductive drum 23 are madegenerally proportional to each other, it is possible to correct theposition when the laser beam L is imaged on the photoconductive drum 23.

Also, the imaging lens 61 is able to correct a deviation of theinclination of the sub-scanning direction of the mutual respectivereflecting surfaces of the polygon mirror 50, namely a deviation of theposition of the sub-scanning direction caused due to scattering in theamount of surface inclination.

Concretely, by making the laser beam incident surface (the side of thepolygon mirror 50) and the emitting surface (the side of thephotoconductive drum 23) of the imaging lens 61 have a generally opticalconjugated relation, even in the case where an inclination definedbetween an arbitrary reflecting surface of the polygon mirror 50 and therotation axis of the polygon mirror 50 defers in every reflectingsurface, it is possible to correct a deviation of the scanning positionof the sub-scanning direction of the laser beam L guided onto thephotoconductive drum 23.

Incidentally, since the cross-sectional beam diameter of the laser beamL replies upon a wavelength of the light beam L emitted from thesemiconductor laser device 41, when the wavelength of the laser beam Lis set up at 785 nm, it is possible to make the cross-sectional beamdiameter of the laser beam L large. Also, by setting up the wavelengthat 630 nm or shorter, it is possible to make the cross-sectional beamdiameter of the laser beam L smaller.

The reflection mirror after the deflection is configured of a planesurface, and correction of the surface inclination is performed only bythe imaging lens 61.

As a matter of course, the surface shape of the imaging lens 61 may be,for example, a toric lens having a rotation symmetrical axis to the mainscanning axis and having a varied curvature of the sub-scanningdirection depending upon the scanning position. According to this, arefractive power of the sub-scanning direction varies depending upon thescanning position, and a scanning line curvature can be corrected.Furthermore, in the case where the curved surface of the sub-scanningdirection has a rotation symmetrical axis, a degree of freedom of thecurvature of the sub-scanning direction is widened, and it is possibleto achieve the correction more precisely.

Also, for example, as illustrated in FIGS. 5( a) and 5(b), imagingmirrors 65-1 and 65-2 having a power may be used in place of the imaginglens 61.

Here, for example, as in an imaging lens 66 as illustrated in FIGS. 6(a) and 6(b), the imaging lens 61 which is included in thepost-deflection optical system 60 may include a surface having adiffraction surface (diffraction optical device). According to this, theinfluence due to an environmental fluctuation can be reduced.Incidentally, in the case of the imaging lens 66 as illustrated in FIGS.6( a) and 6(b), the diffraction surface is provided only in a side ofthe emitting surface, but the diffraction surface may be provided in aside of the incident surface or on the both surfaces. As a matter ofcourse, the same is also applicable in the case where plural imaginglenses are configured. Also, not only the imaging lens but other opticaldevice may be provided.

Also, in general, for example, as in an imaging lens 67 as illustratedin FIGS. 7( a) and 7(b), the diffraction surface is provided on a planesurface. But, for example, as in the imaging lens 66 as illustrated inFIGS. 6( a) and 6(b), by imparting it to a surface with a power, it ispossible to reduce the number of lenses. Furthermore, by bringing apower by a diffraction optical device, it is possible to reduce afluctuation in the wall thickness or to make the wall thickness thin;and it is possible to improve the productivity and the precision and toreduce the cost due to shortening of a molding time.

That is, for example, as illustrated in FIG. 8, by arranging onlycurvatures of lens having a power of the conventional imaging lens 61,it is possible to reduce a fluctuation in the wall thickness and to makethe wall thickness thin while having a lens action. According to this,as in an imaging lens 68 as illustrated in FIG. 9, the fluctuation inthe wall thickness of lens can be reduced. Also, in the case whereplural optical devices after the deflection are configured, the numberof optical devices can be reduced.

Incidentally, a non-illustrated horizontal synchronous sensor isprovided in an opposite side to the polygon mirror 50.

Now, in general, in scanning a luminous flux made emitted from thesemiconductor laser device 41 on the photoconductive drum 23, in theunderillumination scanning optical system, an edge part of the polygonmirror 50 is not used, whereas in the overillumination scanning opticalsystem, an edge part of the polygon mirror is used.

Concretely, for example, as illustrated in FIG. 10( a), in scanning aluminous flux made emitted from the semiconductor laser device 41 on thephotoconductive drum 23, in the underillumination scanning opticalsystem, edge parts x₁ and x₂ of the main scanning direction of thepolygon mirror 50 are not used. However, for example, as illustrated inFIG. 10( b), in scanning a luminous flux made emitted from thesemiconductor laser device 41 on the photoconductive drum 23, in theover-illumination scanning optical system, edge parts y₁ and y₂ of themain scanning direction of the polygon mirror 50 are used.

Here, in the edge parts y₁ and y₂ of the main scanning direction of thepolygon mirror 50, an error in that shape is large. For that reason,when a luminous flux is deflected (scanned) by using the edge parts y₁and y₂, the luminous flux to be deflected (scanned) becomes dull,whereby a wave front aberration is deteriorated in the entire scanningregion on the photoconductive drum 23. As a result, the beam diameter onthe photoconductive drum 23 as an image surface increases, whereby abeam profile is deteriorated.

However, in the case where the imaging lens 61 to be provided in thepost-deflection optical system 60 is an fθ lens configured of a singlelens, it is similarly difficult to correct a wave front aberration inthe entire scanning region on the photoconductive drum 23 because thenumber of surfaces which can be utilized for the correction of the wavefront aberration is only two of an incident surface and an emittingsurface.

In addition, in general, as illustrated in FIG. 2, a dustproof glass 62as a flat plate glass for preventing turnaround of the toner, dusts orpaper powder or the like floating within the image forming section 22into a non-illustrated housing of the optical beam scanning apparatus 21is provided in the post-deflection optical system 60; and on the otherhand, even when it is intended to correct the wave front aberration inthe entire scanning region on the photoconductive drum 23, it isdifficult to correct the wave front aberration in the entire scanningregion on the photoconductive drum 23 by the imaging lens 61 and theflat plate glass because the number of sheet which can be utilized forthe correction of the wave front aberration is only one.

Here, for example, as illustrated in FIG. 11, in general, a flat plateglass 69 is provided as a countermeasure to the noise of the polygonmirror 50 between the polygon mirror 50 and the imaging lens 61particularly in the post-deflection optical system 60.

Then, for the purpose of suitably correcting the wave front aberrationwhile making it easy to correct the wave front aberration in the entirescanning region on the photoconductive drum 23, for example, asillustrated in FIG. 12, the dustproof glass 62 as a flat plate glass isprovided between the imaging lens 61 and the photoconductive drum 23particularly in the post-deflection optical system 60; and at the sametime, the flat plate glass 69 is provided as a countermeasure to thenoise of the polygon mirror 50 between the polygon mirror 50 and theimaging lens 61 particularly in the post-deflection optical system 60.

According to this, it is possible to suitably correct the wave frontaberration while making it easy to correct the wave front aberration inthe entire scanning region on the photoconductive drum 23. Accordingly,it is possible to not only suitably correct the wave front aberration onthe photoconductive drum 23 but obtain suitable beam diameter and beamprofile on the photoconductive drum 23.

FIG. 13( a) shows a simulation result in the case of correcting the wavefront aberration by using a single sheet of flat plate glass (thedustproof glass 62 or the flat plate glass 69) as illustrated in FIG. 2or FIG. 11; and FIG. 13( b) shows a simulation result in the case ofcorrecting the wave front aberration by using the two sheets of flatplate glass (the dustproof glass 62 and the flat plate glass 69) asillustrated in FIG. 12.

Incidentally, in all of FIGS. 13( a) and 13(b), it is shown that thehigher the numerical value on the ordinate, the worse the wave frontaberration is.

For example, as illustrated in FIGS. 13( a) and 13(b), in the case wherethe wave front aberration is corrected by using a single sheet of flatplate glass, the worst wave front aberration is 0.032λ; whereas in thecase where the wave front aberration is corrected by using two sheets offlat plate glass, the worst wave front aberration is 0.028λ. This matterdemonstrates that as compared with the case where the wave frontaberration is corrected by using a single sheet of flat plate glass, inthe case where the wave front aberration is corrected by using twosheets of flat plate glass, the worst wave front aberration becomessmall.

Incidentally, since the flat plate glass 69 which is provided betweenthe polygon mirror 50 and the imaging lens 61 particularly in thepost-deflection optical system 60 as a countermeasure to the noise ofthe polygon mirror 50 deteriorates a transmittance of the luminous flux,a coating may be applied on the flat plate glass 69 for the purpose ofsuppressing the deterioration of transmittance. According to this, it ispossible to more suitably correct the wave front aberration while makingit easy to correct the wave front aberration in the entire scanningregion on the photoconductive drum 23.

In addition, usually, though the dustproof glass 62 is provided atsubstantially 90° against the optical axis between the imaging lens 61and the photoconductive drum 23 particularly in the post-deflectionoptical system 60, and similarly, the flat plate glass 69 is provided ina positional relation of substantially 90° against the optical axisbetween the polygon mirror 50 and the imaging lens 61 particularly inthe post-deflection optical system 60, it is better that the opticalaxis dustproof glass 62 and the flat plate glass 69 are each furtherinclined around the main scanning direction axis (namely, around thelongitudinal direction axis of the photoconductive drum 23 as shown byan arrow M in FIG. 14) against the respective optical axis at a requiredangle. Concretely, for example, as illustrated in FIG. 14, the dustproofglass 62 is further inclined at, for example, 3° around the mainscanning direction axis against the optical axis between the imaginglens 61 and the photoconductive drum 23 particularly in thepost-deflection optical system 60; and the flat plate glass 69 isfurther inclined at, for example, 9.61° around the main scanningdirection axis against the optical axis between the polygon mirror 50and the imaging lens 61 particularly in the post-deflection opticalsystem 60. Incidentally, at this time, the directions to which the twosheets of glass (the dustproof glass 62 and the flat plate glass 69) areinclined are reverse to each other.

In addition, the imaging lens 61 is inclined at, for example, 1.16°around the main scanning direction axis against the optical axis of thepost-deflection optical system 60. According to this, by inclining therespective optical parts, a degree of freedom in correcting the wavefront aberration becomes large, and the wave front aberration can beuniformly reduced with a good balance. Incidentally, in the case of FIG.5, the wave front aberration can also be similarly corrected by usingthe flat plate glass 69, the dustproof glass 62 and the like.

Incidentally, the inclination angle can be properly changed dependingupon the wave front aberration on the photoconductive drum 23 to becorrected.

Incidentally, in the embodiment of the invention, while the wave frontaberration in the entire scanning region on the photoconductive drum 23has been corrected by providing the dustproof glass 62 as a flat plateglass between the imaging lens 61 and the photoconductive drum 23particularly in the post-deflection optical system 60 and simultaneouslyproviding the flat plate glass 69 as a countermeasure to the noise ofthe polygon mirror 50 between the polygon mirror 50 and the imaging lens61 particularly in the post-deflection optical system 60, the inventionis limited to such case. For example, more sheets of flat plate glass,namely three or more sheets of flat plate glass may be provided in thepost-deflection optical system 60. According to this, it is possible tomore suitably correct the wave front aberration while making it easierto correct the wave front aberration in the entire scanning region onthe photoconductive drum 23. Accordingly, it is possible to not onlymore suitably correct the wave front aberration on the photoconductivedrum 23 but obtain more suitable beam diameter and beam profile on thephotoconductive drum 23.

Also, as a matter of course, the invention is also applicable to imaginglenses including a surface having a diffraction surface (diffractionoptical device) (for example, imaging lens 66 to 68 explained withreference to FIGS. 6 to 9).

Furthermore, in the embodiment of the invention, while the flat plateglass (the dustproof glass 62 or the flat plate glass 69) has been usedfor the purpose of correcting a wave front aberration on thephotoconductive drum 23, the invention is not limited to such case, butother flat plates using a resin may be used. That is, so far as it ispossible to correct the wave front aberration on the photoconductivedrum 23, flat plates made of any material may be used.

Incidentally, in the embodiment of the invention, while the inventionhas been applied to the overillumination scanning optical system, as amatter of course, the invention may be applied to the underilluminationscanning system.

Furthermore, the number of a luminous flux from the light source may beone or plural.

1. An optical beam scanning apparatus comprising: a light sourceconfigured to emit one or plural luminous fluxes, a pre-deflectionoptical system configured to form a luminous flux emitted from the lightsource to image the luminous flux as a line image in a prescribeddirection corresponding to a main scanning direction, a scanning unitconfigured to scan the imaged luminous flux by the pre-deflectionoptical system against a scanning subject, a width of the luminous fluxmade incident on the scanning unit from the pre-deflection opticalsystem being wider than a width of one reflecting surface forming thescanning unit, and a post-deflection optical system having at least twosheets of flat plate configured to transmit the luminous flux scanned bythe scanning unit, an optical parts for imaging the luminous fluxscanned by the scanning unit on the scanning subject and a diffractionoptical device in at least one surface included in one of the flat planeand the optical parts.
 2. The optical beam scanning apparatus accordingto claim 1, wherein the flat plate provided in the post-deflectionoptical system is a glass.
 3. The optical beam scanning apparatusaccording to claim 1, wherein at least one sheet of flat plate isprovided between the scanning unit and the scanning subject in thepost-deflection optical system.
 4. The optical beam scanning apparatusaccording to claim 1, wherein the flat plate provided in thepost-deflection optical system is inclined at a required angle around amain scanning direction axis against a prescribed optical axis in thepost-deflection optical system.
 5. The optical beam scanning apparatusaccording to claim 1, wherein at least one sheet of flat plate isprovided between the optical parts and the scanning subject in thepost-deflection optical system.
 6. The optical beam scanning apparatusaccording to claim 1, wherein the optical parts included in thepost-deflection optical system is inclined at a required angle around ahorizontal direction axis against a prescribed optical axis in thepost-deflection optical system in the optical parts included in thepost-deflection optical system.
 7. The optical beam scanning apparatusaccording to claim 1, wherein the optical parts included in thepost-deflection optical system is configured of a single lens.
 8. Theoptical beam scanning apparatus according to claim 1, wherein theoptical parts included in the post-deflection optical system is a lensor a mirror.
 9. An image forming apparatus provided with an optical beamscanning apparatus comprising: a light source configured to emit one orplural luminous fluxes, a pre-deflection optical system configured toform a luminous flux emitted from the light source to image the luminousflux as a line image in a prescribed direction corresponding to a mainscanning direction, a scanning unit for scanning the imaged luminousflux by the pre-deflection optical system against a scanning subject, awidth of the luminous flux made incident on the scanning unit from thepre-deflection optical system being wider than a width of one reflectingsurface forming the scanning unit, and a post-deflection optical systemhaving at least two sheets of flat plate configured to transmit theluminous flux scanned by the scanning unit, an optical parts for imagingthe luminous flux scanned by the scanning unit on the scanning subjectand a diffraction optical device in at least one surface included in oneof the flat plane and the optical parts.
 10. The image forming apparatusaccording to claim 9, wherein the flat plate provided in thepost-deflection optical system is a glass.
 11. The image formingapparatus according to claim 9, wherein at least one sheet of flat plateis provided between the scanning unit and the scanning subject in thepost-deflection optical system.
 12. The image forming apparatusaccording to claim 9, wherein the flat plate provided in thepost-deflection optical system is inclined at a required angle around amain scanning direction axis against a prescribed optical axis in thepost-deflection optical system.
 13. The image forming apparatusaccording to claim 9, wherein at least one sheet of flat plate isprovided between the optical parts and the scanning subject in thepost-deflection optical system.
 14. The image forming apparatusaccording to claim 9, wherein the optical parts included in thepost-deflection optical system is inclined at a required angle around ahorizontal direction axis against a prescribed optical axis in thepost-deflection optical system in the optical parts included in thepost-deflection optical system.
 15. The image forming apparatusaccording to claim 9, wherein the optical parts included in thepost-deflection optical system is configured of a single lens.
 16. Theimage forming apparatus according to claim 9, wherein the optical partsincluded in the post-deflection optical system is a lens or a mirror.